For some time now, the amount of power consumed by servers has been increasing, as has the number of servers placed in various facilities. Servers today consume ten times more power than they did ten years ago. The U.S. Environmental Protection Agency estimated that in 2006 the nation's servers and data centers collectively consumed approximately 61 billion kilowatt-hours, or 1.5% of total U.S. electricity consumption, at a total cost of approximately $4.5 billion. This level represents a doubling in the level of energy consumption by data centers in the five years since 2001; and the level is expected to double again by 2011 as more and more applications shift from the desktop to a server-based computing paradigm. Already there are large server farms comprising a million or more servers. Globally, it has been estimated that the worldwide expenditure on enterprise power and cooling of servers is in excess of $30 billion.
Driving the overall growth in server farms is the increased capacity required to run Internet-based businesses. In addition to the growth in traditional Web-based service providers, a revolution is currently taking place in the $120 billion software industry that will further drive the demand for server resources. Software applications that traditionally have been installed on personal computers are being deployed over the Internet under the software-as-a-service (SaaS) model.
Server farms usually have a fixed peak power budget. This is because large power consumers operating server farms are often billed by power suppliers, in part, based on their peak power requirements. The peak power budget of a server farm also determines its cooling and power delivery infrastructure costs. Hence, companies are interested in maximizing the performance at a server farm given a fixed power budget.